Posted by: Mark Foreman | October 27, 2013

The mechanism of a lithium aluminium hydride reduction

Dear Reader,

It is good to be back, while sadly I can not enter my classroom like “Eyeball Paul” in Kev and Perry go large (I have to use the door like everyone else) I can say that I have a treat for you deserving people.

Now recently one of my readers did not believe me regarding the mechanism of the lithium aluminium hydride reduction of a carbonyl, but I have dug up strong evidence which I hope will be of interest to you as it explains how our fiery friend LAH works.

Now I would like to make an observation about organic chemistry, organic chemists tend to draw curved arrows to explain mechanisms for everything but sometimes they draw a plausible mechanism without doing an experiment to prove it. However in the case of lithium aluminium hydride experiments have been done which prove what the mechanism is.

Now the late and great Ronald Snaith did a lot of very good chemistry where he showed that the binding of lithium to organic molecules is very important. I had the joy of meeting him once when I was a PhD student at an inorganic chemistry conference.

Now some years ago Karl E. Wiegers and Stanley G. Smith published a paper in the Journal of Organic Chemistry (1978, volume 43, page 1126) back in the days when I was in the infants section of primary school. I recall not long after this paper was published I picked up my first science textbook. It was an encyclopedia of science which seemed much more interesting to read rather pay attention to the storys which the deputy headteacher (my form teacher) was reading to the class.

One story which I read / was read about that time was about Marie Curie who was killed by overexposure to radiation, after reading about her untimely death I decided (as a young prechemist) that when I grew up to be a chemist I would do my experiments behind a wall of lead bricks in case I managed to make a radioactive chemical. A while later I discovered that I could not make radioactivity by a mere chemical reaction so I dropped the lead wall idea from my mind. I think that doing chemistry from behind a wall of lead would be much harder, I have had a go at using master slave grabbers of the type used for working with used nuclear fuel and trust me it is not easy to pick up a spanner or a bit of metal pipe with them.

Then a while later as a postdoc when I learnt about the joys of radioactivity from Jiri Hala I learnt about a radioisotope of beryllium whose decay rate is a function of its chemical environment. I will blog about this beryllium radioisotope another day.

Back to the paper of Karl E. Wiegers and Stanley G. Smith, they measured the rate of reaction of camphor with sodium and lithium aluminium hydrides in THF. They found that the sodium salt reacted more slowly which proves that the nature of the cation is important. The sodium cations are less able to bind to the carbonyl oxygen and increase the partial positive charge on the carbonyl carbon.

This provides us with some evidence to explain the reactivity series of lithium > Sodium for the aluminium and boron anionic hydrides.

Further evidence has been provided by the work of two French chemists (Pierre and Handel) who showed that when [2.l.l]cryptand is added to lithium aluminium hydride that it can no longer reduce many functional groups. This is in contrast to the reactivity of enolates where a strong lithium binding agent such as HMPA will make the enolate more reactive. For the enolate the more coordinated the lithium is by the solvent or the additive the more free the anionic enolate is to react with an electrophile. The same is true of many organometallic reagents such as Grignards and organolithiums, for these organometallics the more able the solvent mixture is to coordinate to lithium the more reactive the carbon nucleophile is.

The fact that lithium aluminium hydride shows the opposite trend when the solvent system is made more coordinating to lithium suggests that coordination of lithium cations to the carbonyl is as important (electrophilic nature of the lithium ion) as important as the nucleophilic part (the AlH4 anion) of lithium aluminium hydride.

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Responses

  1. […] The lithium is better able to act as a lewis acid than the sodium, the lewis acid bonds to the carbonyl oxygen. This in turn increases the amount of positive charge density on the carbonyl carbon. As the reduction is favoured by an increase in the positive character of the carbonyl carbon a change from sodium to lithium will make the reducing agent better. For more details please read here. […]

  2. Hi Mark — I have been out of chemistry for almost 30 years (software development, consulting, training, and writing now; http://www.processimpact.com), but it was very interesting to see your reference to one of the papers derived from my PhD thesis (U. of Illinois, 1977). The LAH research was fascinating. I’m glad you found the paper interesting. I do miss chemistry sometimes!

    Karl Wiegers

  3. Dear Karl,

    I am very glad that one of the people who worked out how LiAlH4 and NaBH4 work has been able to drop by on this blog. Feel free to come back and if I do make an error with the chemistry of LiAlH4 then please feel free to point it out.

    I am amazed at how you did a recryst on LiAlH4, your PhD work must have required lots of careful work under inert conditions. I can do inert condition chemsitry but I know it is no walk in the park.

    • Yes, I spent a lot of time with my hands in a glove box or glove bag. Recrystallized LAH is a lovely white crystalline material, not the gray powder that comes out of the can. It’s freely soluble in ether, so the solutions we created with it under argon in a vacuum line were clear and colorless. I’d filter the gray slurry through a sintered glass funnel in a glove box filled with nitrogen. On more than one occasion, the sintered glass simply disappeared with a little *pop* after I took the funnel out of the bag into the air and the residue picked up a bit of moisture. That was startling.

      It’s fun for me to see that I still remember a bit of chemistry after all those decades. My 7th book on software development was published in August (“Software Requirements, 3rd Edition”), so I’ve been away from organic chemistry for a long time.

      • I am from the europe school of thought with organometallics so I would have done it on a vacuum / N2 line using custom made glass, I used to do organometallic chemistry when I was a postdoc in Tony Hill’s group. I only used glove boxes a few times, I have to say I rather hate working in a positive P box for organometallics work. I have worked a little in a negative P box (radioactivity box) and it is much more easy to move your hands in such a box as you are not pushing against the gas in the box.

        I used to use sodium sand a lot for making CyMe4 diketone, when I did this I used to use a German method to dispose of the sodium. I used to put the sodium waste into a bucket of sand with a small hole in the bottom. I then added more sand and then the bucket would be slowly soaked in water. A few times when I was transfering the sodium from the glass filter gadget to the sand bucket I had some flashs of yellow light.

  4. thanx ………


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